Lightguide fiber of the type used for carrying optically coded communication signals is typically fabricated by heating a portion of a lightguide preform in an inductively heated furnace and then drawing a lightguide fiber from that portion of the preform. The preform from which the fiber is drawn comprises a core surrounded by a cladding having a much lower index of refraction than the core. Typically, the furnace in which the preform is heated comprises a set of zirconia rings surrounded by granular zirconia insulation. The granular zirconia insulation is contained in a quartz beaker which is circumscribed by RF powered inductive coils. A copper shell is positioned about the quartz beaker. In operation, the preform is inserted into an opening in the top of the furnace coaxial with the zirconia rings and the fiber is heated and drawn through an opening in the bottom of the furnace which is in vertical alignment with the top opening.
To obtain lightguide fiber having acceptable transmission losses and strength characteristics, the lightguide fiber drawing tension must be carefully regulated. If the lightguide fiber tension falls below an acceptable lower limit, then transmission losses tend to increase. Further, too low a lightguide tension results in increased difficulty in obtaining precise diameter control during drawing because of the lower fiber viscosity. Too high a lightguide fiber tension can undesirably result in a low strength fiber.
The lightguide fiber tension varies with the degree of cooling within the lightguide furnace. Thus, as the lightguide fiber is cooled within the furnace, the viscosity of the drawn fiber increases which, in turn, causes an increase in the fiber tension. With present-day inductively heated lightguide furnaces, the amount of lightguide fiber cooling is generally fixed as a consequence of the relatively small openings at the top and the bottom of the furnace. The top and bottom openings are made small to restrict the entry of contaminants into the furnace. To regulate the degree of lightguide fiber cooling, the furnace temperature itself is regulated by varying the input power to the furnace. Regulation of the furnace input power requires costly and complex electrical controls.
During lightguide fiber fabrication contamination must be avoided which is the reason for restricting the size of the openings into and out of the furnace. Even very minute contaminants will significantly adversely affect lightguide fiber strength if the ocntaminants come into contact with the lightguide fiber during drawing. As an additional precaution against contaminants coming into contact with the drawn lightguide fiber and the lightguide preform, the drawn lightguide fiber and the lightguide preform may be flushed with inert gases directed against the fiber in the manner taught in U.S. Pat. No. 4,030,901 issued June 21, 1977 to Peter Kaiser and assigned to Bell Telephone Laboratories. However, the additional modifications that need to be made to the lightguide furnace to permit the lightguide preform and the lightguide fiber drawn therefrom to be flushed with inert gas increases the furnace complexity and fabrication cost. Further, the flow of inert gases along the fiber may significantly cool the fiber, which may lead to the aforementioned difficulties.
Accordingly, there is a need for a technique to regulate the cooling of the lightguide fiber during drawing to control the fiber tension while minimizing the possibility of contamination thereof.